1. Field of the Invention
The present invention relates to an apparatus for and a method of reproducing a picture signal, and more particularly, to an apparatus for and a method of synchronously reproducing a variety of bit streams, such as audio, video and sub-picture, each having the different system clock reference. The audio, video and sub-picture signals are digital encoded according to a typical coding technology, such as the international standard, ISO-IEC 13818-2, commonly called “MPEG (Moving Picture Expert Group)” and multiplexed to generate a series of digital multiplexed coded bit streams.
2. Description of the Related Art
Recently, a wide variety of digital storage media rapidly become popular. Such digital storage media is typically a mass storage optical memory disk, such as DVD (Digital Versatile Disk) and adapted to record thereon a multiplexed and digital encoded data including a video signal, an audio signal and an additional information signal, such as a subtitle.
In the digital storage media, the video signal is typically encoded according to the MPEG, while the audio signal may be often encoded according to the other standards. The encoded audio and video data may be multiplexed according to a system standard regulated in the MPEG.
Referring to FIG. 4 of the drawings, there is shown an example of the conventional DVD system for reproducing the coded data recorded in the DVD as a picture signal reproducing apparatus 1. As shown in FIG. 4, the picture signal reproducing apparatus 1 is connected to a multiplexed coded stream supplying unit 3, such as a DVD disk drive, and comprises a data transmission controlling unit 5 and a reproduction controlling unit 7. The transmission controlling unit 5 is adapted to transmit the coded data recorded in the DVD from the multiplexed coded stream supplying unit 3. The reproduction controlling unit 7 is adapted to decode the transmitted coded data.
The picture signal reproducing apparatus 1 is linked with an external host computer 9 through a host bus 11. The host computer 9 is adapted to control the data transmission controlling unit 5 and the reproduction controlling unit 7. The host computer 9 has a file system for the DVD disk and its management information. In accordance with this management information transmitted from the host computer 9, the data transmission controlling unit 5 is operated to transmit the coded data from the multiplexed coded stream supplying unit 3 to the reproduction controlling unit 7. The reproduction controlling unit 7 is then operated to decode the coded data on the basis of the coded data transmitted from the data transmission controlling unit 5.
Referring to FIG. 5 of the drawings, there is shown an example of the structure of the digital multiplexed coded streams according to the DVD video standard in the DVD forum. As shown in FIG. 5, the digital multiplexed coded stream includes at least one video object, referred to as “VOB”. Each VOB further includes at least one video object unit, referred to as “VOBU”. Each VOBU further includes a first navigation signal NP and a plurality of video signals VP, audio signals AP, and sub-picture signals SP after the navigation signal NP. Each of the audio, video and sub-picture signals is digital encoded by the encoder, not shown, and then multiplexed for each packet by the multiplexing apparatus, not shown. The digital multiplexed coded streams thus has a series of digital coded data strings including a diversity of information as the audio, video, sub-picture, and so on are multiplexed in order.
Referring to FIG. 6 of the drawings, there is shown an example of the structure of pack and packet as an elementary unit in the typical digital multiplexed coded stream. As shown in FIG. 6, the digital multiplexed coded stream has a plurality of packs 13. Each of packs 13 has a first pack header 14a and at least one packet 14b after the pack header 14a. The pack header 14a includes a system clock reference, hereinlater referred to as simply “SCR”, having information used for regenerating a system time clock, hereinlater referred to as simply “STC”, to decode the multiplexed stream.
Each of packets 14b includes a top packet header 15a and a coded data 15b followed by the packet header 15a. The packet header 15a has a top of synchronous signal 16a, an identifying code 16b, referred to as “stream ID” in the drawings, for identifying a variety of packets, such as video, audio and sub-picture, packet length information 16c, a presentation time stamp 16d, hereinlater referred to as simply “PTS”, and so on. The PTS 16d is indicated when each of reproduction units, i.e., each of packs 13 of the video, audio, and sub-picture is decoded in the reference time and when the coded data 15b is reproduced. The coded data 15b is that video, audio and sub-picture information are coded by the coding apparatus, and contained in that packet 14b corresponding to its stream ID 16b. 
In such DVD system, the reproduction order and synchronous reproduction of the video, audio and sub-picture are controlled in accordance with the SCR and the PIS. In the forward reproduction, the SCR monotonously increases. The SCR sometimes decreases at a connection point between particular two continuous packs, because of the fact that particular stream for the DVD video recording can be edited and rearranged. The DVD forum works out “DVD Specification for Rewritable/Re-recordable Discs. Part3 VIDEO RECORDING” version 1.0 issued on September, 1999, as a particular standard for achieving the video recording in the rewritable types of DVD, such as a DVD-RAM.
Such connection point between two packs having the different SCR is hereinlater referred to as “seamless point”. The seamless point should accurately be detected in order to reset the synchronization of the video, audio and sub-picture at the seamless point.
The host computer 9 can notice the seamless point according to only data stored in the disk. Furthermore, there is another method of detecting the seamless point comprising the steps of: preparing the above information on the seamless point of the host computer 9; decoding the coded stream by the reproduction controlling unit 7 to derive the SCR; and matching the prepared information on the seamless point with the derived SCR to detect the actual seamless point.
Referring to FIG. 7 of the drawings, there is shown a graph of an example of the variation in the SCR between two video object units VOBU0 and VOBU1. As shown in FIG. 7, the SCR of the first video object unit VOBU0 gradually increases at first. After the seamless point, the SCR of the next video object unit VOBU1 may be assumed to be four patterns of variation.
In the patterns of variation on the lines A and B in FIG. 7, the SCR falls down at the seamless point and thereafter gradually rises. In the pattern of variation on the line C, the SCR constantly increases before and after the seamless point. In the pattern of variation on the line D, the SCR rises up at the seamless point and thereafter further gradually rises.
In the aforesaid patterns of variation on the lines A and B, the seamless point can be detected on the basis of the SCR contained in the top of the navigation pack NP for each video object unit VOBU. It is however impossible to detect the seamless point on the basis of these SCR in the aforesaid patterns of variation on the lines C and D. The above variations in the SCR on the lines C and D may occur when the streams are edited and rearranged.
In Japanese Non-examines Patent Publication No. H10-26221 there is disclosed a typical method of reproducing the picture signal on the basis of the seamless point detected according to the SCR in which the picture signal reproducing apparatus may be controlled in accordance with the difference between the last SCR of the former pack and the top SCR of the latter pack. This method may be constructed on the assumption that the top SCR of the latter pack should be smaller than the last SCR of the former pack. For this reason, the seamless point cannot be detected in case of the aforesaid pattern of variation in the SCR on the line D.
According to the DVD Specification for Rewritable/Re-recordable Discs. Part3 VIDEO RECORDING, it is assumed that the last SCR of the pack before the seamless point and the top SCR of the pack after the seamless point may be defined as management information and the seamless point may be detected on the basis of predetermined system clock references for both of the aforesaid packs in the picture signal reproducing apparatus.
The above conventional picture signal reproducing apparatus 1, however, should be operated to transmit the information on the SCR monitored by the host computer 9 for each reproduction unit to the picture signal reproducing apparatus 1 when the stream is transmitted from a prior input buffer, such as the multiplexed coded stream supplying unit 3, to the picture signal reproducing apparatus 1. This process is troublesome in the management.